Electric control system



ELECTRIC CONTROL SYSTEM Filed Nov. 16, 1940 Volta level controlincl/eating device Inventor Theodore Br wn His Attorney- Patented Feb.10, 1942 ELECTRIC CONTROL SYSTEM Theodore R. Brown, Schenectady, N. Y.,assignor to General Electric Company, a corporation of New YorlrApplication November 16, 1940, Serial No. 365,957 .11 Claims. (c1.111-319) My invention relates to electric control sys-' tems and moreparticularly to electric valve control systems for dynamo-electricmachines.

In the provision of excitation circuits for dynamo-electric machines, itis important to incor= porate apparatus which not only maintains anoutput condition, such as the voltage of a machine, at a substantiallyconstant value but also eiiects this control in a rapid and precisemanner. In accordance with the teachings of my inven tion describedhereinafter, I provide a new and improved electric valve control systemfor dynamo-electric machines wherein the output voltage, or otherelectrical condition, is maintained substantially constant, and whichincludes apparatus which controls the electric valve means during thestarting operation. of the machine from an associated alternatingcurrent supply circuit.

It is an object of my invention to provide a new and improved electricvalve control system for dynamo-electric machines.

It is another object of my invention to provide a new and improvedelectric valve voltage regulating circuit for dynamo-electric machines.

It is a further object of my invention to provide a new and improvedelectric valve control system, including an armature reaction excitedmachine,

which controls the excitation of an alternating current dynamo-electricmachine of the synchronous type.

It is a still further object of my invention to provide a new andimproved control and regulating. system for dynamo-electric machines ofthe synchronoustype including an armature reaction excited machine andcomprising means for assuring the transmission oi a constant current tothe field winding of the machine during the starting operation.

Briefly stated, in the illustrated embodiment of my invention 1 providean electric valve regulating system for a dynamo-electric machine of thesynchronous type, such as a synchronous condenser, wherein anarmature-reaction excited machine, or exclter, or sub-exciter, isemployed to maintain an electrical condition, such as the outputvoltage, of the synchronous condenser at a substantially constant value.Apparatus is also provided to assure the transmission of a constantcurrent to the fieldwinding of the synchronous machine during thestarting operation. Additional apparatus is provided to prevent areversal in polarity of the voltage applied to the field winding of thesynchronous condenser and to limit the maximum euergization oi the fieldwinding of the synchronous condenser.

For a better understanding oi my invention, reference may be had to thefollowing description taken in connection with the accompanying drawing,and its scope will he pointed out in the appended claims. The singlefigure of the drawing diagrammatically illustrates my invention asapplied to an excitation system for an alternating current machine ofthe synchronous type.

Considering now more particularly the single figure of the drawing, myinvention is there illustrated as applied to an. excitation system ioian alternating current dynamo-electric machine of the synchronous type,such as a synchronous condenser l including armature'winding 2 and afield winding 3. The armature winding 2 is connected to a suitablealternating current circuit such as a polyphase alternating currentcircuit ii.

I may employ an exciter 5 comprising an armature winding 6 whichenergizes the field winding 3 of synchronous condenser i, and which alsoincludes a field winding 71' which controls in the particular embodimentshown serves as a sub-exciter.

The machine E is provided with an armature winding 9 and is alsoprovided with two sets of brushes; one set of brushes including brushesiii armature reaction excited machine and ii are considered as theprimary brushes and are short circuited or connected through a conductorof low resistance to provide a path for the short circuit current whichproduces one component oi the exciting fiux of the machine. The otherset of brushes including brushes i2 and i3 produce a voltage whichvaries as the resultant of the magnetic field produced bythe boostcontrolwinding l4 and the buck control winding 15. The output orsecondary brushes l2 and I3 are connected to the field winding! ofexciter 5 which constitutes a load circuit'f orfthe 8. "Theirmaturewinding 9 of machine 8 may be connected to field winding 1 throughresistances I- and I1, the latter 01' which may be adjustable topreestablish or preset the range of excitation of exciter 5.

Switches l3 and I5 may also be employed to close or open selectively thearmature winding circuit of machine 5. A contactor (not shown)-including contacts 20, 2| and 23 may be employed to control theoperation of the machine 3 so that it may be either manually orautomatically operated. When the contactor is in the position such thatcontacts 20, 2| and 23 are in the positions shown, the system is incondition for automatic operation, and when the contacts are moved tothe other positions, that is when contact is closed, contact 2| is openand when contact 23 is open, the system is in condition for manualoperation. When in the last mentioned positions, these contacts permitthe manual operation or control of the output voltage of the exciter 5by means of resistance III. A suitable switch, such as a push button 22,may be employed as a means for temporarily efiecting energization orexcitation of the control windin s of machine 3.

Electric translating apparatus, including electric valve means 24 and 25is employed for variably energizing boost control winding i5,

thereby controlling the excitation and output voltage of the armaturereaction excited machine 3. The electric valve means 24 and 25 areprererably of the high vacuum type each comprising an anode 23, acathode 21, a control grid 23, a screen grid 29, and a suppressor grid33 which is preferably connected to cathode 21. As a source oiunidirectional current ior energizing control windings l4 and |5, Iprovide a rectifier circuit 3| including transformers 32 and 33, havingprimary windings 34 and 35 and secondary windings 33 and 31,respectively. The primary windings 34 and 35 are Scott or T-connected,and the secondary windings 35 and 31 are connected to suitable rectifiervalves 33, 39 and 40, 4| respectively. The output voltage of therectifier valves 33- are connected to be additive and are connected tocontrol windings l4 and I5 through a smoothing inductance 42. Afiltering capacitance 43 may be connected across the output circuit ifdesired. The anode-cathode circuits of the electric valve means 24 and25 are connected to the boost control winding |4 through conductors 44and 43, so that the variation in the efifective impedance of electricvalve means 24 and 25 controls the current transmitted to this winding.On the other hand, the buck control winding |5 is connected directlyacross the output circuit of the rectifier 3| through a circuitincluding conductors 44 and 43 and including a resistance 41, so thatsubstantially constant current is transmitted to this winding. As ameans for controlling the conductivities of the electric valve means 24and 23 in a manner to compensate for variations in the output voltage ofrectifier 3| occasioned by variations in the alternating current circuit4. I provide a circuit including a resistance 43 connected between thepositive terminal or the out put circuit or rectifier 3| and the screengrids 23.

I employ a control means 43 for the translating apparatus includingelectric valve means 24 and 25. This control means may take the form ofa voltage regulating circuit such as that disclosed and claimed in acopending patent application Serial No. 274,765 01 Jerry L. Stratton.filed May 20,1939, and which is scanned to the assignee of the presentapplication. The control means comprises a rectifier circuit whichproduces a unidirectional voltage which varies in response to thevoltage of circuit 4 and which includes transformers 53 and 5| havingprimary windings 52 and 53 and secondary windings 54 and 55,respectively. The primary windings are Scott-connected or T-connected,and the rectifier valves 55, 51 and 55, 55 are connected to secondarywindings 54 and 55, respectively, and in a manner so that the outputvoltages are additive in accordance with the features disclosed andclaimed in the above-identified copending patent application 8. N.274,765. A filter circuit 55, including capacitances 5| and inductances52, may be connected across the output. circuit of the rectifierapparatus to eliminate the ripple voltage.

The control means 45 also includes means ior producing a variableunidirectional control voltage which is impressed on the control grids23 of electric valve means 24 and 25, which in turn, control theresultant excitation of the armature reaction excited machine 5. Thesource of unidirectional current produced by transformers 53 and 5| andelectric valves 55--59 is connected to suitable impedance elements, suchas resistances 54 and 35 which are connected in series relation withelectric discharge devices 35, 51 and suitable constant voltage devicessuch as glow discharge valves 55 and 59. Electric discharge devices 55and 51 and glow discharge valves 33 and, respectively, constituteparallel circuits and are employed in order to assure continuity 01service or the system in the event either one 01 the electric dischargedevices 55 or 51 becomes defective or inoperative. Electric dischargedevices 55 and 31 are preferably 01 the high vacuum type, eachcomprising an anode 10, a cathode 1|, a control grid 12, a screen grid13 and a suppressor grid 14 which is preferably connected to theassociated cathode 1|. The glow discharge valves 58 and 55, when in aconducting condition, serve to maintain the potentials of the cathodes1| substantially constant, thereby increasing the sensitivity or theregulating system. Suitable voltage dividers including resistances 15and 13 are connected between the positive terminals 01 the glowdischarge valves 53 and 55, respectively, and the positive terminal orthe direct current source provided by rectifier valves 55-55. By

virtue oi the adjustable connections 11 and 13 a predetermined componentof the voltage of the direct current source is impressed on the screengrids 13 so that the conductivity or the current conducted by theelectric discharge devices 33 and 31 varies in response to the magnitudeor the direct current circuit or the voltage of the alternating currentcircuit 4.

As a means for adjusting or preestablishing the conductivities of theelectric discharge devices 35 and 51 so that these discharge devicesconduct equal amounts of current, I provide a voltage divider includingresistances 13, 30 and 5| having contacts 52 and 33 which are connectedto control grids 12 oi electric discharge devices 33 and 31 throughresistances 34 and 35,- respectively. In addition, this voltage dividerimpresses on the control grids 12 components oi-voltage which vary inresponse to the output voltage or the. rectifiers and which also vary inresponse to the magnitude or the voltage of circuit 4, so that theconductivities of electric discharge devices 33 and 31 vary in responseto these voltages. The variable unidirectlonal voltage produced byresistances 84 and 65 and electric discharge devices 66 and 81 isimpressed on control grids 28 of electric valve means 2t and 25 througha circuit including a resistance 85, conductor 81, conductor 88,resistance 89 and a voltage divider including resistance 99.

In order to assure continuity of service in the event one of theparallel paths of the control means 49 becomes defective, the controlmeans 49 is adjusted so that either one of the parallel paths willassume control of the system in case the other is unable to contributeits part to the operation. More particularly, the resistances 63 and I5and 16 are designed so that if one of the glow discharge valves, such asdischarge valve 68, becomes open circuited or defective, the reductionin current through resistance 63, that is through the left-hand portionof resistance 63, will assure a rise in potential of tap it so that thecurrent drawn through resistances 64 and 65 becomes substantiallyconstant for a par= ticular value of voltage applied to the regulatingmeans 49. It will be appreciated that the system will operate in asimilar manner if the other parallel circuit, including glow dischargevalve 69, becomes defective.

An anti-hun ting circuit comprising a transformer 95 is connected to beresponsive to the rate of change of the output or armature voltv age ofmachine 5 and to impress a compensating voltage on the control grids E2of electric discharge devices 56 and lil through valves 68, 69,resistances l5 and it; and resistances ti] and 84, respectively.

In order to prevent reversal of the armature or output voltage ofmachine 5, I provide a cir cult including unidirectional conductingdevices 82 and 93 connected between the armature or output circuit ofmachine 5 through a circuit including an adjustable resistance as andresistance to. If the armature voltage tends to reverse, theunidirectional conducting devices $2 and til conduct current to producepositive voltages across resistances 8t and. 6d, tending to increase thecurrent conducted by electric valve means 2d and '25 and therebyincreasing the net or resultant excitation of machine In addition, Iprovide unidirectional conducting devices 95 and 963 which arepreferably of the high vacuum type comprising control grids 9?. Theseunidirectional conducting devices are controlled in response to thepolarity of the armature or output voltage of machine by having controlgrids ill connected to the armature circuit through a resistance 98.Upon being rendered conductive, unidirectional conducting devices tiland lit conduct current through resistance 89 to produce an increasednegative voltage which is impressed on the cathodes El by electric valvemeans 2% and 25, thereby increasing the current conducted. In thismanner, the net or resultant excitation of the machine 8 is furtherincreased, tending to prevent a reversal in polarity of the outputvoltage of machine 5.

To provide a means for limiting the maximum energizatlon of the controlwinding l4, and, hence, to limit the maximum or output voltage ofmachine 5, I provide unidirectional conducting devices 99 and ltd whichare connected to limit the maximum value of positive voltage which maybe impressed on control grids 28 of electric valve means 26% and '25.The unidirectional conducting devices 98 and, Hill may be of the typecomprising a pair of anodes l0! and I02 and cathodes lit. The cathodesM3 may be connected to the armature circuit of machine 8 through avoltage divider including a resist.

ance I04 and resistances M35 and its. These resistances may he providedwith adjustable taps to control the potentials oi the cathodes M3,thereby providing means for adjusting or controlling the currentconducted by these parallel operating electric valves. The tap onresistance m5 also establishes the exciter voltage required before theoverload regulation introduces the additional droop, characteristic. Thetap on resistance it also establishes the value to which the excitervoltage will he limited by the field temperature control efiectedthrough contactor lit described hereinafter. anodes ml are com nectedtogether and are connected to the anodes ll! of electric dischargedevices 35 and 61 through ans adjustable resistaruze mi and a resistanceA switching means, such as a contactor mils connected between thearmature circuit of ma-- chine 5 and the control means :39 to impress onthe control grids 2c of electric valve means it and 25 a predeterminedcomponent of the armature voltage so that the output or armature voltageof machin 5 remains constant and, which in turn, transmits asubstantially constant field current to synchronous condenser 8. Whenthe contactor its is closed, a predetermined variable component ofvoltage is impressed on the control grids 28 of electric valve mean itand 25 so that a constant current is transmitted to the ileld winding 3of machine l. The contactor ltd may be operated by suitable apparatus(not shown) and may be operated in response to starting apparatus forthe synchronous condenser i, so that a constant current is transmittedto the field winding 3 of condenser l during a period of the startingsequence oi operation. A further switch or contactor Mil may beconnected in the circuit including unidirectional conducting devices soand ltd to limit the maximum value of field current transmitted tosynchronous condenser i. Contactor till may be actuated by apparatus(not shown) such as a field temperature responsive or indicating devicewhich is actuated in response to the temperature of the field winding ofsynchronous conden er 6.

Suitable means such as a'resistance ill may be connected between thesource of variable unidirectional vol-tage provided Icy the rectifiersin-- cluding transformers tit and 5t and valves till-59 and electricvalves 66 and ill in order to establish or preset the voltage level orthe magnitude of the voltage which is maintained in alternating currentcircuit t. Resistance iii may be adjustable so that the voltage which ismaintained by the'regulating system isadjustable or controllable. g I

The operation of the embodiment of my invention shown in the singlfigure will be explained by considering the system when it is operatingto control the excitation of synchronous condenser l in response to thevoltage of the alternating current circuit 4.

Variable amounts of field current are transmitted to field winding 23 ofsynchronous condenser I of exciter 5. The current transmitted to fieldwinding l of exciter ii is determined by the difference or the sum ofthe armature voltage of exciter 5 and the armature voltage of thearmature reaction excited machine 8. The output voltage of the armaturereaction excited machine 8 may either oppose or assist the armaturevoltage of machine i and thereby controls the current transmitted tofield winding I. Of course, the output voltage of the machine I issubstantially less than the armature voltage of machine 5.

The output or armature voltage of machine I is determined by thedifference in the magnetometive forces produced by the boostfieldwinding II and the buck control winding ll. Control winding II, inasmuchas it is connected across a source of substantially constant voltage,produces a constant magnetomotive iorce, whereas the magnetomotive forceof th boost control winding 14 is varied in response to the voltage ofcircuit 4. The resultant or diflerence in the magnetomotive forcesdetermines the output voltage or current or machine 8. Switch is is, orcourse, closed so that the armature of machine is connected to variablyenergize field winding I of exciter I.

' Control means 49 impresses a variable unidirectional voltage oncontrol grids 28 electric valve means 24 and 25 to control the currentconducted by these electric valve means. This unidirectional voltagevaries in response to the voltage of circuit 4 and the manner in whichthat voltage is produced will now be considered. The magnitude of theunidirectional voltage produced by the rectifiers, includingtransformers 50 and and rectifier valves 56-59, varies in response tothe magnitude of the polyphase voltages oi circuit 4. Electric dischargedevices It and l transmit variable amounts of unidirectional currentthrough resistances 64 and 85, the conductivities of these dischargedevices being controlled by the variable unidirectional voltagesimpressed on control grids 12 and screen grids 13. This unidirectionalvoltage produced by the variable amounts of unidirectional currenttransmitted throughresistance 84 is impressed on control grids 2! ofelectric valve means 24 and 25. For example, if the voltage of thealternating current circuit 4 rises abov the predetermined orpreestablished value, electric discharge devices Cl and 61 conduct anincreased amount or current to increase the voltage drop acrossresistance '4. Consequently, the potential impressed on control grids 28of electric valve means 24 and 2! is decreased and the current conductedby these electric valve means is correspondingly decreased, eilecting areduction in the current transmitted to the boost control winding l4 ofmachine I. As a result, the excitation of machine I is reduced and theoutput voltage thereof and the excitation of exciter 5 and synchronouscondenser I are correspondingly reduced, tending to restore the voltageof circuit 4 to the desired value. If the voltage of the alternatingcurrent circuit 4 tends to fall below th preestablished value, thereverse operation takes place eiiecting an increase in the value ofcurrent transmitted to field winding 8 and tending to raise the armatureor output voltage oi. condenser I to the desired value.

The system also prevents reversal oi the armature or output voltage ormachine I. This operation is obtained by virtue of the unidirectionalconducting devices 92 and Q3 and the unidirectional conducting devices95 and 86. If the output or armature voltage of machine I should tend toreverse, unidirectional conducting devices 22 and 83 conduct currentthrough resistances N and 86 tending to raise the potential of controlgrids 22 of electric valve means 24 and 25, thereby increasing theenergization of control winding l4 and thereby preventing a reversal ofexcitation. This action is assisted by the operation of theunidirectional conducting devices I and N which, as the armature voltagetends to reverse, are rendered conducting thereby transmitting a currentthrough resistance It and producing thereacross a negative voltage whichtends to lower the potential of the cathodes 21 of electric valve means24 and 25, thereby further increasing the current conducted by theseelectric valve means and tending to increase still further theenergization of control winding l4. In this manner, it is assured thatthe output voltage 0! machine will not be reversed in polarity.

Iithecontactor III is closedduetotherisein temperature of field windingI by virtue of operation of apparatus not shown, indicating that themaximum condition iield temperature has been attained, the potentialbetween the positive terminal of the exciter bus and the tap onresistance III will not be permitted to rise more than the voltage dropacross resistance 85. This action limits the exciter armature voltage toa predetermined value.

In the event the load imposed on synchronous condenser I, as indicatedby the excitation or armature voltage, tends to rise above apredetermined value, the unidirectional conducting devices as and Hillconduct current through resistances i4 and which is proportional to theamount of the overload, thereby limiting the maximum voltage which isimpressed on the control grids 28 of electric valve means 24 and 25.

when contactor i0! is closed, the control means 42 is renderedsubstantially ineilective and a predetermined component of the armaturevoltage or machine 5 is employed to control the conductivities ofelectric valve means 24 and 25 and. hence, control or maintain thearmature voltage of machine 5 at a predetermined value. By holding thearmature voltage at a definite value a predetermined constant fieldcurrent will be transmitted to the field winding I of synchronouscondenser I. Upon closure of contactor Ill a predetermined component ofarmature voltage 0! machine 5 is impressed on control grids 28 ofelectric valve means 24 and 25 through the circuit including resistance94, contactor I", resistance i5, resistance 88, conductor 81, conductor88, resistance 88 and portions of resistances ii and II. In this manner,the above described circuit operates as a voltage controlling element tomaintain the armature voltage of exciter I at a substantially constantvalue. when the armature voltage tends to rise, the voltage impressed ongrids 22 is lowered; and conversely, when the armature voltage tends todecrease the grid voltage rises. This operation is accomplished by usingthe variable voltage drop acrossresistanccs 85 and It.

It should be noted that the control means ll is energized through acircuit independent of the rectiflers which energize the electric valvemeans 24 and 25, so that the control means is not influenced byvariations in control current transmitted to boost winding [4. It willbe appreciated that it is important to vary the current transmitted tothe field winding [4 very rapidly. Consequently, in order to obtainprecision of control, it has been found desirable to separate these twoparts of the system.

Inasmuch as the buck control winding II is connected directly across theoutput circuit of rectifier 3|, it is desirable to increase the currentto the boost control winding i4 upon an increase in the energization ofwinding II occasioned by a temporary rise in voltage oi circuit 4. Acoming H is accomplished by means of the variation in voltage impressedon the screen grids 29 through resistance 48 which automaticallyincreases the current transmitted to the boost control winding H as theoutput voltage of rectifier 3| increases. In this manner, theenergization of winding i4 is automatically varied to compensate forchanges in the energization of winding [5 occasioned by variations inthe output voltage of rectifier 3|.

While I have shown and described my invention as applied to a particularsystem of connections and as embodying various devices diagrammaticallyshown, it will be obvious to those skilled in the art that changes andmodifications may be made without departing from my invention, and I,therefore, aim in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In combination, adynamo-electric machine comprising an armaturewinding and a field winding, an alternating current circuit connected tosaid armature winding, electric apparatus for controlling theenergization of said field winding and comprising an armature-reactionexcited dynamo-electric machine including a control winding, electrictranslating apparatus connected between said alternating current circuitand said control winding and including an electric valve means forcontrolling the current transmitted to said control winding, saidelectric valve means including a control member, a control meansconnected to said alternating current circuit for variably energizingsaid control member, and switching means for rendering ineffective saidcontrol means by impressing av voltage onsaid control member so thatconstant current is transmitted to the field winding of the firstmentioned dynamo-electric machine.

2. In combination, a dynamo-electric machine comprising an armaturewinding and a field winding, an alternating current circuit connected tosaid armature winding, electric apparatus for controlling theenergization of said field winding and comprising an armature-reactionexcited dynamo-electric machine including a control winding, electrictranslating apparatus connected between said alternating current circuitand said control winding and including an electric valve means forcontrolling the current transmitted to said control winding, saidelectric valve means including a control member, a voltage controllingcircuit connected to said alternating current circuit for impressing avariable voltage on said control member to maintain the voltage of saidalternating current circuit substantially constant, and switching meansfor rendering inefiective said voltage controlling circuit comprisingmeans for impressing on said control member a voltage so that constantcurrent is transmitted to the field winding of the first mentioneddynamo-electric machine.

3. In combination, a dynamo-electric machine comprising an armaturewinding and a field winding, an alternating current circuit connected tosaid armature winding, electric apparatus for controlling theenergization of said field winding and comprising an armature-reactionexcited dynamo-electric machine including a control wind- I ing,electric translating apparatus connected between said alternatingcurrent circuit and said control winding and including electric valvemeans for controlling the current transmitted to said control winding,said electric valve means including a grid, a voltage regulating circuitconnected to said alternating currentcircuit and comprising an amplifierfor variably energizing said grid, and switching means for effectivelyshunting said regulating circuit to impress a voltage on said grid 50that constant current is transmitted to the field winding of the firstmentioned dynamo-electric machine.

4. In combination, an alternating current circuit, a dynamo-electricmachine having an armature winding connected to said circuit andincluding a field winding, electric apparatus for controlling theenergization of said field winding and comprising an armature-reactionexcited machine including a control winding, control means connected tosaid alternating current circuit and responsive to an electricalcondition thereof and comprising electric ,valve means for variablyenergizing said control winding, said electric valve means having acontrol member for determining the current conducted thereby, meansresponsive to said control means iorimpressing a variable voltage onsaid control 'member to control the current transmitted to said controlwinding, and switching means connected between said control means andsaid control member for renderingsaid control means inefiective byimpressing on said control member a variable voltage to assure thetransmission of a substantially constant current to the field winding ofsaid dynamo-electric machine.

5. In combination, an alternating current circuit, a dynamo-electricmachine having an armature winding and a field winding, the armaturewinding being connected to said alternating current circuit, electricapparatus for controlling the energization of said field winding andcomprising a dynamo-electric machine of.

the armature-reaction excited type and including a control winding,electric translating-apparatus connected between said alternatingcurrent circuit and said control winding and including electric valvemeans having a grid, a voltage responsive circuit connected to saidalternating current circuit and comprising a serially connectedimpedance device and electric discharge device for producing aunidirectional voltage which varies in response to the magnitude of thevoltage of said alternating current circuit, an amplifier connectedbetween the voltage regulating. circuit and said grid, and meansconnected between said voltage responsive circuit and said amplifier forrendering said voltage responsive circuit ineffective by impressing avariable voltage on said grid so that constant cur rent is transmittedto the field winding of the first mentioned machine.

6. In combination, a load circuit, a dynamoelectric machine of thearmature reaction excited type for energizing said load circuit andcomprising a control winding, electric translating apparatus forenergizing said control winding and including electric valve meanshaving acontrol grid, control means for impressing a variable voltage onsaid control grid comprising a source of direct current and a seriallyconnected impedance element and an electric discharge device, andswitching means for shunting said control means to maintain the voltageof the load circuit constant.

'7. In combination, a load circuit, apparatus including adynamo-electric machine of the armature reaction excited type comprisingan armature winding connected to said load circuit and having a controlwinding, means for supplying variable amounts oi unidirectional currentto said control winding to control the current transmitted to said loadcircuit, control means for said last mentioned means, and meansconnected between said armature circuit and said control meanscomprising a unidirectional conducting device for controlling saidcontrol means to prevent reversal in polarity oi the voltage oi saidload circuit.

8. In combination, a load circuit, apparatus ineluding a dynamo-electricmachine of the armature reaction excited type comprising an armaturewinding connected to said load circuit and including a control winding,electric valve means for controlling the energlzation of said controlwinding and comprising a control member, control means for impressing avariable voltage on said control member to control the currenttransmitted to said load circuit, and means connected between said loadcircuit and said control means for controlling said control means toprevent reversal in the polarity of the voltage applied to said loadcircuit.

9. In combination, a load circuit, apparatus including a dynamo-electricmachine of the armature reaction excited type comprising an armaturewinding connected to said load circuit and including a control winding,electric translating apparatus comprising an electric valve means fortransmitting variable amounts unidirectional current to said controlwinding, said electric valve means including a control member, con- 1trol means for impressing a variable voltage on said control member andcomprising a sourc of direct current and a serially connected impedanceelement and an electric discharge device for transmitting variableamounts of an unidirectional current through said impedance element, anda unidirectional conducting device connected between said control meansand said load circuit for preventing reversal in polarity of the voltageapplied to said load circuit.

10. In combination, a load circuit, a direct current dynammelectricmachine of the armature reaction excited type comprising an armaturewinding connected to said load circuit and including a control winding,supply means (or transmitting variable amounts of unidirectional currentto said control winding to control the current transmitted to said loadcircuit, control means for said supply means, means comprising aunidirectional conducting device connected between said load circuit andsaid control means to prevent reversal in polarity of the voltage ofsaid load circuit, and means comprising a unidirectional conductingdevice connected between said load circuit and said control means tolimit the maximum amount of current transmitted to said control winding.

11. In combination, a load circuit, apparatus including adynamo-electric machine of the armature reaction excited type comprisingan armature winding connected to said load circuit and having a pair ofopposing control field windings, means for supplying a substantiallyconstant current to one of said field windings, apparatus for supplyinga variable current to the other field winding, control means for saidapparatus, and a unidirectional conducting device connected between saidload circuit and said control means for preventing reversal in polarityof the voltage applied to said load circuit.

caravan 12. In combination, a load circuit, apparatus including adynamo-electric machine oi the armature reaction excited type comprisingan armature winding connected to said load circuit and having a pair oi.opposing control field windings, means for supplying a substantiallyconstant current to one or said field windings, apparatus for supplyinga variable current to the other field winding, control means forproducing a variable voltage for controlling said apparatus, and meansfor controlling said voltage to prevent reversal in polarity oi thevoltage oi said load circuit comprising a unidirectional conductingmeans connected between said load circuit and said control means.

13. In combination, a load circuit, apparatus including adynamo-electric machine 0! the armature reaction excited type comprisingan ar mature winding connected to said load circuit and having a pair ofopposing control field windings, means for supplying a substantiallyconstant current to one of said field windings, electric translatingapparatus including an electric valve means for transmitting variableamounts oi unidirectional current to the other field winding, saidelectric valve means having an anode, a cathode and a control grid,means for impressing a variable voltage on said grid, and meanscomprising a unidirectional conducting means connected between said loadcircuit and said con trol means to prevent reversal in polarity of thevoltage supplied to said load circuit.

14. In combination, a load circuit, apparatus including adynamo-electric machine of the armature reaction excited type comprisingan armature winding connected to said load circuit and having a pair 01'opposing control field windings, means for supplying a substantiallyconstant current to one of said field windings, electric translatingapparatus including an electric valve means for transmlting variableamounts of unidirectional current to the other field winding, saidelectric valve means having an anode, a cathode and a control grid,control means for impressing on said grid a variable unidirectionalvoltage the magnitude or which determines the amount oi. currenttransmitted to said other field winding, and means including aunidirectional conducting device connected between said load circuit andsaid control means for preventing reversal in polarity of the voltage oisaid load circuit.

15. In combination, a load circuit. apparatus including adynamo-electric machine oi the armature reaction excited type comprisingan armature winding connected to said load circuit and having a pair ofopposing control field windings, means for supplying a substantiallyconstant current to one of said field windings, electric translatingapparatus including an electric valve means for transmitting variableamounts oi unidirectional current to the other field-winding, saidelectric valve means having an anode, a cathode and a control grid,control means for impressing on said grid a variable unidirectionalvoltage to control the amount of current transmitted to said other fieldwinding, means including a unidirectional conducting device connectedbetween said load circuit and said control means to control saidunidirectional voltage to prevent reversal in polarity of the armaturevoltage of said machine by raising the potential or the grid relative tothe cathode, and a second means for preventing reversal in polarity ofth armature voltage or said machine comprising a controlled electricdischarge device responsive to the armature voltage of said machine forlowering the potential of said cathode.

16. In combination. a direct current load circuit. a dynamo-electricmachine of th direct current type having an armature winding connectedto said load circuit and having a field winding, a dynamo-electricmachine of the armature reaction excited type having an armature windingconnected to said field winding and having a control winding, means forvariably energizing said control winding to control the armature voltageof the first mentioned machine, and means for controlling theenergization of said control winding to prevent reversal in polarity ofthe armature voltage of the first mentioned machine.

1'7. In combination, a direct current load circuit, a dynamo-electricmachine of the direct current type having an armature winding connectedto said load circuit and having a field winding, means for variablyenergizing said field winding comprising an armatur reaction excitedmachine connected in circuit with said field winding and the armaturewinding of the first mentioned machine and comprising a control winding,means for variably energizing said control winding so that the armaturevoltage of said armature reaction excited machine aids or opposes thearmature voltage of the first mentioned machine to control theenergization of said field winding, and means for controlling the lastmentioned means to prevent reversal in the polarity of the armaturevoltage of said first mentioned machine.

THEODORE R. BROWN.

